Electrophysiological and biochemical methods will be employed to investigate the association between acetylcholine (ACh) secretion and the release of adenosine triphosphate (ATP) and derivatives from motor nerve endings in vertebrate skeletal muscle. Alterations in quantal ACh output will be produced by changing the ionic composition of the bathing medium (e.g., adding La3 ion, substituting Ba2 ion for Ca2 ion) and the release of ATP and derivatives will be assayed by the firefly luciferase method. An attempt will be made to determine if bare nerve ending preparations release ATP and derivatives. In addition, the relative contribution of presynaptic and postsynaptic sources to nucleotide release will be determined. Finally, the mechanism of the inhibition by adenosine compounds on ACh release will be analyzed in detail.